Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons.
The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates which can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids which have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide is used, which may or may not be crosslinked. The thickened or gelled fluid helps keep the proppants within the fluid during the fracturing operation.
While polymers have been used in the past as gelling agents in fracturing fluids to carry or suspend solid particles in the brine, such polymers require separate breaker compositions to be injected to reduce the viscosity. Further, the polymers tend to leave a coating on the proppant even after the gelled fluid is broken, which coating may interfere with the functioning of the proppant. Studies have also shown that “fish-eyes” and/or “microgels” present in some polymer gelled carrier fluids will plug pore throats, leading to impaired leakoff and causing formation damage. Conventional polymers are also either cationic or anionic which present the disadvantage of likely damage to the producing formations.
Aqueous fluids gelled with viscoelastic surfactants (VESs) are also known in the art. However, under some conditions, the VES-gelled fluids will separate and/or precipitate over time, for instance at high temperatures. It would be desirable if a composition and method could be devised to stabilize and enhance aqueous VES-gelled fluids used in injection of treatment fluids such as fracturing fluids.